Ageing of the DGEBA/TETA epoxy system with off-stoichiometric compositions

Abstract

An investigation was carried out on the room temperature ageing of off-stoichiometric DGEBA/TETA epoxy formulations. The results obtained show that the epoxy rich mixtures have their inherent brittleness increased by the ageing treatment due to recrystalization of the unreacted epoxy monomers, although homopolymerization could also play a minor role. The initial reaction steps dominated by the amine addition reactions control the macromolecular structure and the mechanical performance of the stoichiometric and near stoichiometric formulation with excess of epoxy monomer. Plasticization due to absorbed -OH results on a significant increase of the deformability of these formulations. The amine rich mixtures have the more stable structures, although plasticization due to moisture absorption from the surrounding environment also produces an increase on the deformability of all, but one, of the formulations investigated. keywords: epoxy resins, ageing, mechanical propertiesoff-stoichiometric mixtures. For the particular system made of the triethylene tetramine, TETA, hardener and the DGEBA monomer the variation of the hardener to monomer ratio promotes strong changes on the mechanical behavior [18][19][20][21] . Of particular interest with respect to these changes is the very sharp increase in the impact strength, when amine rich mixtures were used 21 . The problem of working with off-stoichiometric mixtures is that latent reaction sites could remain on the macromolecular structure developed and under the proper conditions the structure can evolve, resulting in changes on the mechanical performance of the material. Temperature is clearly one external parameter that could cause changes to the system, but, even at room temperature, ageing due to exposure of the epoxy resin to humid environments could also be important [22][23][24] . The changes observed on the mechanical properties when the epoxy monomer to hardener ratio is varied, and the possible changes due to aging, are a direct consequence of the different macromolecular structures that are developed and/or the possible reactions that could occur given a

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